Add site announcement banner

[Yassinbrine]

/claim archestra-ai#4463

Summary

• add organization fields and migration metadata for one site-wide announcement with optional expiration
• expose announcement content through appearance settings and add settings controls for markdown content plus expiration
• add a right-hand app-shell banner that renders markdown links only when unexpired and when the user has siteAnnouncement:read
• add siteAnnouncement RBAC defaults and regression coverage for storing and returning announcement fields

Tests

• Not run locally: pnpm is not available in this shell environment.

Note: I attempted to open this against archestra-ai/archestra, but GitHub blocks new PRs there from non-prior contributors. This fork PR is based on the upstream main commit 04323e0497126d73c89217507045320a14a6f31e for a clean review diff.